Electrodeposition of copper

ABSTRACT

A copper pyrophosphate electroplating bath contains a known heterocyclic additive in conjunction with an auxiliary brightener selected from iminodiacetic acid, cinammic acid, aliphatic acid, di- and poly- carboxylic acids having at least seven carbon atoms, salts of the aforesaid acids and hydroxyethylcellulose. The auxiliary brightener prevents step plating which is normally caused by the presence of the heterocyclic brightener.

& llmte States Patent [151 3,674,660

Lyde July 4, 1972 [54] ELECTRODEPOSITION OF COPPER [56] References Cited[72] Inventor: Derek Martin Lyde, Stourbridge, England UNITED STATESPATENTS 73 Assignee; m & Wilson Limited, Oldbury near 2,195,409 4/1940Flett ..204/49 X Birmingham, England 3 3/1943 2,700,019 1/1955 I22]hled. Aug. 14, 1970 116L575 IZHQM 3,341,433 9/l967 [2| I Appl. No:63,954

Primary ExaminerG. L. Kaplan Related Apphcamn Data Attorney-Herbert H.Goodman [63] Continuation-impart of Ser. No. 724,225, April 25,

1968. [57] ABSTRACT 30 Foreign A fi fl P -i it Data A copperpyrophosphateelectroplating bath contains a known heterocychc additivein conjunction with an auxiliary y 1, 1967 Great Bmam 1 brightenerselected from iminodiacetic acid, cinammic acid, aliphatic acid, diandpolycarboxylic acids having at least i seven carbon atoms, salts of theaforesaid acids and hydrox- 58] g [52 R 52 44 1 yethylcellulose. Theauxiliary brightener prevents step plating which is normally caused bythe presence of the heterocyclic brightener.

13 Claims, No Drawings ELECTRODEPOSITION OF COPPER This application is acontinuation-in-part application of copending application Ser. No.724,225, filed Apr. 25, 1968, now abandoned. v

The present invention relates to improvements in the electrodepositionof copper.

It is known to carry out the electrodeposition of copper from alkalinesolutions containing an alkali metal copper pyrophosphate complex of theformula X,Cu( 2 where X represents an alkali metal.

A characteristic feature of these plating solutions is that inconjunction with suitable brightening additives they may be used to formhighly specular mirror bright finishes which do not require subsequentmechanical polishing. In this they differ from the conventional acidcopper plating systems and from copper cyanide systems, which producematt finishes. Brighteners used in the former solutions are intended toimprove the specularity of the bright finish whereas those used in thelatter systems are designed to make the matt finish more amenable tomechanical polishing.

Various heterocyclic compounds have been proposed as brighteneradditives for alkaline pyrophosphate electroplating compositions andsome of these have found particular favor in the art. A group of theseheterocyclic compounds is characterized by the presence of the grouping:

contained in a fiveor six-membered heterocyclic ring system where Qrepresents a nitrogen atom (either as a =-=Nor N-grouping) or a sulphuratom. Examples of these mercaptoheterocyclic compounds are described inUK. Specifications Nos. 939,997; 940,282 and 1,051,150.

Some of the aforementioned specifications refer to the optional presencein the electroplating composition of other organic compounds, such assimple aliphatic or hydroxyaliphatic carboxylic acids including oxalic,citric, tartaric, acetic, propionic and phthalic acids and wettingagents. Certain of these compounds are stated, for example inspecification No. 1,051,150, to improve the grain refinement of theelectrodeposited material and to reduce the anodic polarization. Theseadditives were originally described in the early work on copperpyrophosphate baths, before the advent of heterocyclic brighteners. Inpractice, however, we have discovered that although these knowncarboxylic acid additives and wetting agents caused some slightimprovements in the unbrightened copper pyrophosphate baths for whichthey were originally proposed, they do not produce any noticeable effectin the presence of heterocyclic brighteners. This may be because theheterocyclic brighteners introduce such a striking improvement in thespecular brightness and appearance of the work as completely to overideany effects of these previously known additives. Certainly theseadditives, such as citric acid, have not in commercial practice beenused in conjunction with the more successful heterocyclic brighteners.

The heterocyclic brighteners have one disadvantage-they tend to cause aplating defect called step plating. This arises when in consequence ofreduction of current density across the surface of the workpiece, suchas commonly occurs in practice with contoured surfaces, the thickness ofthe coating varies. in normal plating there may be a continuousvariation of the thickness of the coating across the surface withoutaffecting the appearance or quality of the work. In step plating thereis a sharp division between an area of relatively thick plating of highspecular brightness and an area of relatively thin plating of inferiorquality. This tendency can be avoided by keeping the concentration ofheterocyclic brightener small, but that imposes limitations on theamount by which the specular brightness is improved.

I have discovered that certain compounds may be used as auxiliarybrighteners in conjunction with heterocyclic brighteners to providehigher levels of specular brightness with negligible tendency to stepplating. An object of my invention is to inhibit the tendency ofheterocyclic brighteners to cause step plating in copper pyrophosphateelectroplating baths. A further object of my invention is to providecopper pyrophosphate electroplating baths which produce work of improvedspecular brightness. A further object of my invention is to providecopper pyrophosphate electroplating baths of improved levelling power.

The invention provides, in brightening compositions for addition tocopper pyrophosphate electroplating baths and containing a heterocyclicbrightener selected from the known group consisting of2-mercaptothiazole, 2-mercap tobenzthiazole, Z-mercaptothiadiazole,Z-mercaptopyrimidine, Z-mercaptoiminazole, substituted homologs of theaforesaid heterocyclic brighteners and derivatives that form any of theaforesaid compounds when dissolved in copper pyrophosphateelectroplating baths, the improvement which consists in that thebrightening composition contains, in addition to the heterocyclicbrightener, an auxiliary brightener selected from the group of stepplating inhibitors consisting of aliphatic dicarboxylic andpolycarboxylic acids having at least seven carbon atoms, malonic acid,cinammic acid, iminodiacetic acid,salts of the said acids andhydroxyethyl-cellulose.

The invention further provides an aqueous electrolyte for copper platinghaving dissolved therein a copper salt, an alkali metal pyrophosphate inan amount at least sufficient to form the complex X Cu(l O-,) where Xrepresents an alkali metal, and a minor proportion of a brighteningcomposition according to the invention. The invention also provides aprocess for the electrodeposition of copper using an aqueous electrolyteof the invention.

The heterocyclic brighteners for present use are exemplified by themercapto-thiazole compounds described in specification No. 940,282, forexample 2-mercapto-1, 3- thiazole and 2-mercapto-benthiazole; by theZ-mercapto- 1,3,4-thiadiazole compounds described in specification No.939,997, for example 2,5-dimercapto-l,3,4-thiadiazole, 2-mercapto-5-methylmercapto-1,3,4-thiadiazole and2-mercapto-5-n-butylmercapto-l,3,4-thiadiazole; and by the2-mercapto-iminazole and Z-mercapto pyrimidines described inspecification No. 1,051,150, for example Z-mercapto-lmethyliminazole,2-mercaptopyrimidine, 6-hydroxy-2 mercaptopyrimidine and6-hydroxy-2-mercapto-4-methylpyrimidine. Also included among the classesof brighteners are precursors of the compounds described above. Byprecursor is meant herein a compound which when dissolved in the copperpyrophosphate electrolyte provides in solution a compound containing thestructure (II). For example there may be used mercapto salts, orheterocyclic disulphides containing the grouping:

(III) The auxiliary brighteners for present use are for the most partacids which will normally be used as sodium, potassium, or ammoniumsalts.

There may be used, usually in the form of water-soluble salts, organicdiand poly-carboxylic acids or anhydrides thereof having more than sevencarbon atoms. As examples of these compounds there can be mentionedsuberic acid, azelaic acid, and sebacic acid as well as adducts of thetype formed when a dienophilic carboxylic acid such as maleic anhydrideis condensed with a conjugally unsaturated hydrocarbon such aspolyiso-butylene, for example the alkylor alkenyl-substituted succinicacids and anhydrides wherein the alkyl or alkenyl group contains from30-150 carbon atoms.

Normally the heterocyclic brightening agent is employed in a proportionof at least 0.001 grams per liter of the total weight of electrolyte andpreferably from 1 to mg. per liter. The optimum concentration may varyfrom 2 to 4 mg. per liter depending upon the nature of the auxiliarybrightener present. Normally the auxiliary brightener is employed in aproportion of from 1 ppm to saturation, preferably from 2 to 100 ppm,for example 3 to 6 ppm.

It is preferred that the copper plating electrolytes of the inventionhave a similar constitution with respect to the concentration of coppersalt and pyrophosphate as those conventionally employed, for example asdescribed in the abovementioned specifications. These electrolytes maycontain other additives that are conventionally employed in this type ofelectrolyte in addition to the heterocyclic brighteners. The electrolytemay be employed for plating metal articles in accordance with knownprocedures.

The invention is particularly surprising because the compounds whichhave been found effective are similar chemically to a number ofcompounds which had already been tested and found totally inefiective.These ineffective compounds include oxalic acid, tartaric acid, citricacid, formic acid, acetic acid, propionic acid, benzoic acid andphthalic acid.

The invention is illustrated by the following examples wherein therewere employed electrolytes of the following constitution:

Copper pyrophosphate 94 g/l, potassium pyrophosphate 300 g/l, ammonia 1g/l. Standard solutions were made by using 3 heterocyclic brighteners,namely 4 ppm. 2,5-dimercapto- 1,3,4-thiadiazole, 6 ppm.Z-mercaptobenzimidazole and 10 ppm.Z-mercaptobenzthiazole. The solutionswere obtained at a pH of 8.8 at a temperature of 55 C. Plating wascarried out on brass panels in a standard l-lull cell with a meancurrent density of 30 amp per sq. ft. under air agitation.

Tests were repeated with the three standard electrolytes containing thefollowing auxiliary brighteners at a concentration of 10 ppm;

. Cinnamic acid Aurine tricarboxylic acid Hydroxyethyl celluloselminodiacetic acid Malonic acid Sebacic acid Azelaic acid Suberic acidMaleic anhydride/polyisobutylene adduct (Na salt) 10. Oxalic acid 1 l.Citric acid 12. Tartaric acid 13. Formic acid 14. Acetic acid 15.Propionic acid 16. Benzoic acid 17. Phthalic acid Examples 1 to 9provided excellent specular brightness with improved leveling andnegligible step plating in each of the electrolytes. ComparativeExamples 10 to 17 on the other hand, although providing high specularbrightness in regions of high current density, gave rise to serious stepplating.

It was found that Examples 1 to 9, when tested in an electrolytecontaining no heterocyclic brightener, either gave no detectableimprovement, or else were actually disadvantageous.

We claim:

1. In a copper pyrophosphate electroplating electrolyte consistingessentially of an alkaline aqueous solution containing dissolved thereina copper salt and an alkali metal pyrophosphate in an amount at leastsufiicient to form the complex salt X Cu(P O-,),, and a knownheterocyclic brightening additive in an amount sufficient to providebright electrodeposits selected from the group consisting ofmercaptothiazoles, mercaptobenzthiazoles, mercaptothiadiazoles,mercaptopyrimidines, and mercaptoiminazoles, the improvement whereinsaid electrolyte also contains an auxiliary brightener selected from thegroup of step plating inhibitors consisting of imino diacetic acid,malonic acid, cinnamic acid, aurine tricarboxylic acid, aliphaticdicarboxylic acids having at least seven carbon atoms, salts of theaforesaid acids and hydroxyethyl cellulose in a concentration effectiveto inhibit step plating.

2. The electrolyte of claim 1 wherein said heterocyclic brightener isselected from the group consisting of 2-mercaptothiazole,2-mercaptonbenzthiazole, 2-mercaptothiadiazole, 2-mercaptopyrimidines,and Z-mercapto iminazole, in a concentration of from 1 to 10 ppm, andwherein said auxiliary brightener is in a concentration of from 2 toppm.

3. The electrolyte of claim 2 wherein said auxiliary brightener is anadduct of maleic acid with a polyolephine having from 30 to carbonatoms.

4. The electrolyte of claim 3 wherein said polyolephine ispolyisobutylene.

5. The electrolyte of claim 2 wherein said auxiliary brightener issebacic acid.

6. The electrolyte of claim 2 brightener is azelaic acid.

7. The electrolyte of claim 2 brightener is suberic acid.

8. The electrolyte of claim 2 brightener is cinnamic acid.

9. The electrolyte of claim 2 brightener is aurine tricarboxylic acid.

10. The electrolyte of claim 2 wherein brightener is hydroxyethylcellulose.

11. The electrolyte of claim 2 wherein brightener is iminodiacetic acid.

12. The electrolyte of claim 9 wherein brightener is malonic acid.

13. In a process for electrodepositing bright copper from an aqueouscopper phyrophosphate electrolyte wherein copper is electrodepositedfrom electrolyte consisting essentially of an alkaline aqueous solutioncontaining dissolved therein a copper salt and an alkali metalpyrophosphate in an amount at least sufiicient to form the complex saltX Cu(P O and of a known heterocyclic brightening additive in an amountsufficient to provide bright electrodeposits selected from the groupconsisting of mercaptothiazoles, mercaptobenzthiazoles,mercaptothiadiazoles, mercaptopyrimidines, and mercaptoiminazoles, theimprovement comprising admixing in said electrolyte at least one partper million of an auxiliary brightener selected from the group of stepplating inhibitors consisting of imino diacetic acid, malonic acid,cinnimac acid, aurine tricarboxylic acid, aliphatic dicarboxylic acidshaving at least seven carbon atoms, salts of the aforesaid acids andhydroxyethyl cellulose.

wherein said auxiliary wherein said auxiliary wherein said auxiliarywherein said auxiliary said auxiliary said auxiliary said auxiliary

2. The electrolyte of claim 1 wherein said heterocyclic brightener isselected from the group consisting of 2-mercaptothiazole,2-mercaptonbenzthiazole, 2-mercaptothiadiazole, 2-mercaptopyrimidines,and 2-mercapto iminazole, in a concentration of from 1 to 10 ppm, andwherein said auxiliary brightener is in a concentration of from 2 to 100ppm.
 3. The electrolyte of claim 2 wherein said auxiliary brightener isan adduct of maleic acid with a polyolephine having from 30 to 150carbon atoms.
 4. The electrolyte of claim 3 wherein said polyolephine ispolyisobutylene.
 5. The electrolyte of claim 2 wherein said auxiliarybrightener is sebacic acid.
 6. The electrolyte of claim 2 wherein saidauxiliary brightener is azelaic acid.
 7. The electrolyte of claim 2wherein said auxiliary brightener is suberic acid.
 8. The electrolyte ofclaim 2 wherein said auxiliary brightener is cinnamic acid.
 9. Theelectrolyte of claim 2 wherein said auxiliary brightener is aurinetricarboxylic acid.
 10. The electrolyte of claim 2 wherein saidauxiliary brightener is hydroxyethyl cellulose.
 11. The electrolyte ofclaim 2 wherein said auxiliary brightener is iminodiacetic acid.
 12. Theelectrolyte of claim 9 wherein said auxiliary brightener is malonicacid.
 13. In a process for electrodepositing bright copper from anaqueous copper phyrophosphate electrolyte wherein copper iselectrodeposited from electrolyte consisting essentially of an alkalineaqueous solution containing dissolved therein a copper salt and analkali metal pyrophosphate in an amount at least sufficient to form thecomplex salt X6Cu(P2O7)2, and of a known heterocyclic brighteningadditive in an amount sufficient to provide bright electrodepositsselected from the group consisting of mercaptothiazoles,mercaptobenzthiazoles, mercaptothiadiazoles, mercaptopyrimidines, andmercaptoiminazoles, the improvement comprising admixing in saidelectrolyte at least one part per million of an auxiliary brightenerselected from the group of step plating inhibitors consisting of iminodiacetic acid, malonic acid, cinnimac acid, aurine tricarboxylic acid,aliphatic dicarboxylic acids having at least seven carbon atoms, saltsof the aforesaid acids and hydroxyethyl cellulose.